the left side and the rest of cartridge shimano BB's are steel , there are aluminum replacements for the right side.
aftermarket.
But really It just stabilizes the sleeve of the assembly..
perhaps you overtightened it when you put it in.?

the preponderance of plastic BB cups makes me queasy. Here're some musings on the subject:

-Get an all-metal BB set next time. Some of em are dirt-cheap. All the new shimano square taper BB sets seem to be plastic on the left side, which is sad.
-make sure you don't overtighten or cross-thread the bb cup, regardless of material
-someone mentioned loctite. This stuff could end up effing with the plastic.
-if you're BB is exceedingly difficult to install, you may want to get the shell re-tapped.

It was overtightened when installed. It should not be a problem even if cracked if it stays firmly in the bottom bracket. It functions as a round shim/support on the left side. Shimano used to make an aluminium replacement, but I have not been able to find those for some time.

that maybe the case. Should I just return it and stick with loose BB? I like it better because it gives you peice of mind that its full metal. Or should I go with the superior design of seal cartridge? i already asked for a new cup and i'll see if i can install this one correctly.

Not wearing a helmet makes me more aware of my surroundings. I find myself anticipating the hardness of concrete 50 or 100 feet in front of me, it's almost a zen-like connection between my face and the pavement.